PROJECT SUMMARY Taxanes, (microtubule stabilizers), are the only chemotherapy class that improves survival of patients with prostate cancer (PC). Data from two recent PC clinical trials demonstrated unprecedented survival advantages for men treated with docetaxel given at the time of standard androgen deprivation therapy (ADT). The survival benefit achieved by the combination of docetaxel+ADT is the highest ever seen in PC, significantly longer than any prior phase III trial with any therapy, including surgery. While these trials have changed the standard of care in PC, not all men respond equally. Biomarkers of taxane clinical response/resistance do not exist and the molecular mechanisms underlying taxane clinical activity are poorly understood. We were the first to demonstrate that taxanes inhibit the nuclear trafficking and transcriptional activity of the androgen receptor (AR) downstream of microtubule stabilization and that certain AR splice variants (AR-V) confer drug resistance (AR-v7) or sensitivity (AR-v567), based on whether or not they contain the hinge domain which we identified as the microtubule binding domain of AR. We showed that taxane treatment keeps AR inactive in the cytoplasm in preclinical models and clinical samples. We also discovered that ERG expression, which occurs in at least 50% of PC as a result of recurrent gene fusions, mediates taxane resistance by a novel mechanism that involves alteration of microtubule dynamics and homeostasis. Using clinical samples retrospectively and CTCs in real-time, we confirmed the association between ERG expression and taxane resistance. Our long-term goal is to identify clinically relevant and actionable pathways of resistance to taxane chemotherapy so that we can truly achieve precision medicine and enable treatment customization to the individual. The objective here is to identify mechanisms of taxane resistance by determining the impact of AR-V and ERG expression on PC patient response to taxane chemotherapy. To achieve this objective, we propose to use existing CTC samples prospectively collected at baseline, on treatment and relapse, in the context of a multi-Institutional, prospective Phase II trial exploring molecular markers of response to taxanes in men with metastatic castration-resistant PC (mCRPC). Utilizing the already collected CTC samples we plan to 1) Determine the impact of AR-V and ERG expression on response/resistance to taxane chemotherapy; and 2) Identify additional clinically meaningful mechanisms of taxane resistance by performing untargeted RNA sequencing of both pre-treatment and relapse CTC samples. This proposal is expected to yield clinically relevant findings in the short-term. The establishment of an easy to obtain biomarker (i.e. a blood sample) that might lead a physician to guide a patient along a certain treatment path would be extraordinarily useful. In addition, identifying new mechanisms of resistance will allow researchers to develop strategies to minimize resistance and/or develop new drugs.